At the present time, several bifocal contact lenses are available for public use but these are not considered very successful by the eyecare professions (see Bennett et al, 1990). Most present bifocal contact lenses may be divided into two major types and several alternate designs of each type based on the location of the optical zones. Optical zones are those regions of the lens which provide an optical correction for good vision at specified distances for the wearer.
The two major types of bifocal contact lenses are the concentric zone type and the vertically separated zones or segment type as described in U.S. Pat. No. 3,684,357 and Josephson 1990. Each bifocal lens type may be formed of either hard or soft materials. The concentric type is the oldest form of bifocal contact lens. It has two optical zones which are placed concentrically in a bullseye fashion. Depending on the lens design either the distance or near optical zone may be in the most central portion of the lens which is surrounded concentrically by the alternative of the distance or near portion. The optical zones are produced by having two different radii of curvature on either the anterior or posterior lens surface.
The concentric bifocal lens usually functions by what is known as the simultaneous vision principle. To accomplish this, the optical portion of the lens must have such dimensions that, when worn on the eye, both the distance and near optical zones cover a portion of the wearer's entrance pupil so that light passing through both distance and near zones will contribute to the retinal image. The disadvantage of the simultaneous image principle is that the retinal image is composed of light from both distance and near objects of view simultaneously and the image is never completely clear. When the subject looks at a distant object the light from near objects which passes into the entrance pupil forms an out-of-focus veiling effect on the retinal image and when the subject looks at near objects there is the same type of veiling effect from the distance light. This retinal veiling, sometimes called fogging, or blur, has severely limited the ability of most subjects to tolerate a simultaneous vision type of lens correction, although in a few circumstances the patient may learn to ignore the veiling image and use the lens with a limited degree of success. Vision may also be enhanced if some translating movement of the lens occurs to shift the lens either up and down or sideways so that the subject sees through a greater proportion of either the distance or near optical zone. Unfortunately, the shifting action of the lens is usually insufficient or unreliable to be effective in producing the necessary lens translation to move the lens back and forth in order that the wearer may see alternatively between distance and near optical zones to accomplish the alternating form of visual correction.
Another design of the concentric bifocal contact lens consists of a lens with aspherical curves on either the front or back surface as revealed in U.S. Pat. No. 3,482,906. The back surface of the lens has a varying radius of curvature which, beginning in the geometric center of the lens, has the radius needed to produce the contact lens power for the eye's correction for its distance refractive error and changes to radii that are longer and longer in lens positions towards the periphery. This lens has the advantage that there is no sharp transition between the distance and near optical zones of the lens, but rather a continuous optical power change in going from the center to the periphery of the lens. The aspheric bifocal lens suffers from the same drawbacks as other concentric lens designs, however, in that when worn on the eye, there is a variation of optical power of the lens for light which passes into the subject's entrance pupil, which produces a retinal image that does not have the optimal focus for the individual. An aspheric bifocal may function more effectively if the lens can be made to shift as the subject looks from distance to near, but it is an exceptional patient when this occurs reliably so that the patient can see well at both far and near distances. As an alternate design, an aspheric bifocal can be produced by forming an aspherical curve on the front surface of the contact lens, such lens functioning on the eye in essentially the same way as when the aspherical curve is on the posterior surface.
A novel variation of the concentric zone bifocal is the diffraction bifocal design, which utilizes a series of concentric surface zones in the form of a Fresnel half-wavelength zone plate as revealed in U.S. Pat. No. 4,210,391; U.S. Pat. No. 4,641,934 and U.S. Pat. No. 4,655,565. This lens is sometimes termed a "full aperture bifocal" and has been made from both RGP and hydrogel materials. The principle of diffraction has considerable advantage over refraction, in that it requires no appreciable lens thickness which is greater than single-vision designs.
The mechanism for the deviation of light by diffraction requires a fine facet or eschelet only a few .mu.m high on the posterior lens surface which asymmetrically retards transmitted light such that it is in phase at the near focal point. The finer the structure, the greater the deviation. The rulings on a contact lens take the form of concentric circles, and the greater deviation angle required at the periphery of the lens means that the separation between the circles becomes less towards the periphery. By careful selection of the zone widths, it is possible to manipulate the light mainly into two images and create a simultaneous vision contact lens.
In contrast to the two-zone bifocals, the diffractive bifocal always has many zones covering the area of the pupil, and hence the division of the incident light into two images occurs at every small area of the lens. Consequently, as the pupil changes size, the proportion of light for the distance and near remains constant. In addition, this ratio is constant for various patients with different pupil diameters.
The principle disadvantage of the diffractive bifocal is that out-of-focus light is always superimposed on the image that is in focus. Hence, when the patient looks at distance, the out-of-focus light from near objects produces a blurring or hazing of the visual field. In this respect, the lens suffers from the same drawback of simultaneous vision as a two-zone bifocal, although less hazing effect is claimed. Most complaints occur when the lens is worn at night.
The second major type of bifocal contact lens is the vertically stacked zone lens or segmented bifocal as described in U.S. Pat. No. 3,597,055. This lens design requires some method of stabilizing meridional orientation of the lens when worn on the eye so that the distance optical zone stays at the uppermost position of the lens, a design feature which is usually accomplished by the use of prism ballast. The lens is generally fitted so that its lower edge will rest upon the lower lid when the wearer looks at a distant object. As the eye looks down to view a near object, the cornea moves downward relative to the lower lid with the result that the lens is pushed upward by the lower lid. This positions the lower optical zone of the lens, which contains the optical correction for near vision, in front of the pupil. This type of lens depends upon translation of the lens to the proper position whenever the eye gazes from distance to near or returns. This lens often fails because of the lack of adequate movement of the lens to bring the near optic zone to a position in front of the entrance pupil, when viewing near objects. In addition, the lens often fails because of discomfort which may be due to the thick lower edge of the prism ballast design or other methods which are used to establish meridional orientation. In general, mechanical forms of bifocal lens shifting by the lids have been too unreliable for a high degree of success.
Previous bifocal contact lens designs have utilized the lids to move all or part of the contact lens so as to position the desired distance or near optical zone in front of the entrance pupil at the desired time such as revealed in U.S. Pat. No. 4,302,081; U.S. Pat. No. 4,614,413 and U.S. Pat. No. 4,728,182. A contact lens has also been revealed in U.S. Pat. No. 4,702,573 in which the lid induces a change in lens shape to produce an optical power change.
It should be noted that all of the bifocal lens designs which have been described have been composed of either rigid or flexible materials including PMMA, silicone methacrylate, fluorosilicone acrylates, fluoropolymers, silicone resin, silicone elastomer, hydrogel contact lenses of water contents ranging from 30 to 90% (it would be no different for water contents outside this range), butylacrylates and all similar materials known to have properties which allow the construction of a contact lens. My following invention will also apply to lenses made from all of these materials as well as other materials suitable for making contact lenses of any rigidity.